I agree with Les that temperature might be the factor, but this just occured to me (and if I overlooked the answer in a previous post, I apologize).
Are you using tap water? If so, that may be the cause (or one of them).
Minerals and contaminants in your tap water might react with the dektol, and such contaminants/minerals may not be constant throughout the year... therefore, although the powdered dektol you use is the same, the water you mix it in is not. Just something to keep in mind.